The present invention is in the field of vertical underground boreholes and vertical underground cut-off walls.
Methods for the construction of watertight cut-off walls in the ground are already known, for example for the construction of cut-off walls, which eliminate the necessity of expensive draining and sealing operations during excavations, especially in alluvial grounds. For example, see Veder U.S. Pat. No. 2,791,886, incorporated herewith by reference.
Prior art cut-off walls do not always ensure the desired water tightness. The reason for this is partly because it is difficult to obtain a perfect interlocking or interengagement of the single elements, partly because ordinary methods of drilling by means of boring tubes do not guarantee a perfect verticalness of the final wall so that supplementary injection operations are necessary in order to consolidate the wall and to fill in the empty spaces.
The purpose of a cut-off wall for a dam is to intercept the venues of seepage and eliminate them by constructing a permanent positive barrier.
The barrier should have the capacity of accommodating small movements which will occur during the raising and lowering of the reservoir level, thus it should preferably be built with plastic concrete.
Two major concerns must be addressed by the methodology of constructing the wall:                stability of the excavation when seepage areas are encountered;        continuity and minimum section of the wall at the joints.        
The first concern can be solved by utilizing a secant-wall method, which minimizes the size of the open excavation and enhances its stability by the arching effect of the round holes.
The second concern is harder to satisfy, but the problem is solved by using the double stage drilling system of the present invention.
The invention features the following:
A preferred embodiment of the system comprises using directional drilling techniques, commonly utilized in the oil field industry, to drill a relatively small (e.g. 12¼″) hole to full depth of the cut-off wall with a proven accuracy of ¼ degree at 10,000′. At the cut-off wall depth of 700′ for example, this translates to less than ½″. Even accounting for instrumentation and operator errors, it is safe to assume that the bottom of the hole will be within a one foot diameter of the true center.
The small hole is drilled with steerable mud motor/drills, using bentonite as fluid. Once the small hole is drilled and stabilized with a proper mud system, the drill string will be extracted and a casing of 12″ or less will be run to full depth of the smaller hole.
This will act as the guide for the drilling of a much larger (e.g. 36″-48″) hole (e.g. depending on field tests), using a doughnut shaped bit. The drilling of the larger hole be done under bentonite mud using either direct or reverse circulation.
It is important to note the limited size of the open hole, drilled in a ground already partially stabilized by the 12¼″ previously drilled hole.
When this drilling is completed, the outer drill string is withdrawn and the plastic concrete is pumped from the bottom up using the 12″ (or less) casing as the tremie pipe.
This wall will be constructed by alternating a series of primary piles and secondary secant ones; the diameter and spacing will be determined by field tests which will verify the accuracy of the drilling method.
A second preferred embodiment of the invention features a method of constructing an underground vertical hole in a selected location to a depth exceeding 100 feet comprising,
(a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding hole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, drive mechanisms for the drill outer string,
(b) lowering the drill string assembly in an optional starter casing and set it vertical,
(c) advance the inner drill string to the full depth using the steerable mud motor/drill to assure verticality of said hole,
(d) extracting the inner drill string and installing an inner casing to the full depth,
(e) advance the outer drill string to said full depth using the inner casing as a verticality guide for said outer drill string.
A method of constructing an underground wall along a selected path and having predetermined deep depth comprising,
(a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, and drive a mechanism for the outer drill string,
(b) advance the inner drill string to the full predetermined depth of the wall using the steerable mud motor/drill to assure verticality to the full depth of said wall,
(c) extracting the inner drill string and installing an inner casing to the full depth,
(d) advance the outer drill string to the full depth of said wall using the inner casing as a verticality guide for said outer drill string,
(e) remove the drill string assembly and using the inner casing as a tremie pipe, fill the hole with a wall forming plastic concrete to form a first vertical wall element,
(f) repeat the process along the selected wall path at a distance less than the diameter of the previously constructed element,
(g) repeat the process in between the two previously completed elements, cutting into them to assure continuity of the wall.
Apparatus for excavating an underground borehole having a predetermined deep depth comprising,
a drill string assembly having outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, respectively, and a separate drive head for said outer drill string.
A method of constructing an underground vertical hole in a selected location to a depth exceeding 100 feet comprising,
(a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, drive mechanisms for the drill outer string,
(b) install a vertical starter casing at the selected location (optional),
(c) lowering the drill string assembly in the starter casing and set it vertical,
(d) advance the inner drill string to the full depth using the steerable mud motor/drill to assure verticality of said hole,
(e) advance the outer drill string to said full depth using the inner drill string as a verticality guide for said outer drill string.
A method of constructing an underground wall along a selected path and having predetermined deep depth comprising,
(a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, and drive a mechanism for the outer drill string,
(b) install a vertical starter casing in the path of the wall,
(c) lowering the drill string assembly in the starter casing and set it vertical,
(d) advance the inner drill string to the full depth of the wall using the steerable mud motor/drill to assure verticality to the full depth of said wall,
(e) advance the outer drill string to the full depth of said wall using the inner drill string as a verticality guide for said outer drill string,
(f) remove the drill string assembly and fill the hole with a wall forming material to form a first vertical wall element,
(g) repeat the process along the selected wall path at a distance less than the diameter of the previously constructed element,
(h) repeat the process in between the two previously completed elements, cutting into them to assure continuity of the wall.
A method of constructing a deep underground wall along a selected path comprising,
(a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, said inner drill string having a steerable mud motor/drill at the lower end thereof, said outer drill string having one or more drill bits at the lower end thereof, and a drive mechanism for said outer drill string,
(b) install a vertical starter casing in the path of the wall,
(c) lowering the drill string assembly in the starter casing and set it vertical,
(d) advance the inner drill string for a predetermined distance using the steerable mud motor/drill to assure verticality to the full depth of said wall,
(e) remove the drill string assembly and fill the hole with a wall forming material to form a first vertical wall element,
(f) at a predetermined distance along the path of said wall repeating steps (b)-(g) to form a second vertical wall element,
(g) excavating the earth between said first and second wall elements using said first and second elements as a guide to form a panel slot there between and filling said panel slot with a wall forming material, and
(h) repeating steps (b)-(i) at least one further time positioned along the selected path of the wall at a distance less than the diameter of the drill holes,
(i) create the connection between the two panels by forming a circular element in between them.
A method of constructing an underground vertical hole in a selected location to a depth exceeding 100 feet comprising,
(a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, each drill string having one or more drill bits at the lower end thereof, respectively, and drive mechanisms for the drill strings,
(b) install a vertical starter casing at the selected location,
(c) lowering the drill string assembly in the starter casing and set it vertical,
(d) advance the inner drill string for a predetermined distance using the central guiding borehole in the outer drill string as a verticality guide,
(e) advance the outer drill string for said predetermined distance using the inner drill string as a verticality guide,
(f) repeat steps (c)-(e) to final depth,
A method of constructing an underground wall along a selected path and having predetermined deep depth comprising,
(a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, each drill string having one or more drill bits at the lower end thereof, respectively, and drive mechanisms for the drill strings,
(b) install a vertical starter casing in the path of the wall,
(c) lowering the drill string assembly in the starter casing and set it vertical,
(d) advance the inner drill string for a predetermined distance using the central guiding borehole in the outer drill string as a verticality guide,
(e) advance the outer drill string for said predetermined distance using the inner drill string as a verticality guide,
(f) repeat steps (c)-(e) to final depth,
(g) remove the drill string assembly and fill the hole with a wall forming material to form a first vertical wall element,
(h) repeat the process along the selected wall path at a distance less than the diameter of the previously constructed element,
(i) repeat the process in between the two previously completed elements, cutting into them to assure continuity of the wall.
A method of constructing a deep underground wall along a selected path comprising
(a) providing a drill string assembly comprised of outer and inner drill strings with the outer drill string having a central guiding borehole for telescopingly receiving the inner drill string, each drill string having one or more drill bits at the lower end thereof, respectively, and drive mechanisms for the drill strings,
(b) install a vertical starter casing in the path of the wall,
(c) lowering the drill string assembly in the starter casing and set it vertical,
(d) advance the inner drill string for a predetermined distance using the central guiding borehole in the outer drill string as a guide,
(e) advance the outer drill string for said predetermined distance using the inner drill string as a guide,
(f) repeat steps (c)-(e) to final depth,
(g) remove the drill string assembly and fill the hole with a wall forming material to form a first vertical wall element,
(h) at a predetermined distance along the path of said wall repeating steps (b)-(g) to form a second vertical wall element,
(i) excavating the earth between said first and second wall elements using said first and second elements as a guide to form a panel slot there between and filling said panel slot with a wall forming material, and
(j) repeating steps (b)-(i) at least one further time positioned along the selected path of the wall at a distance less than the diameter of the drill holes,
(k) create the connection between the two panels by forming a circular element in between them.
A wall constructed in accordance with a further feature of this invention consists of a series of substantially vertical cylindrical elements which intersect, interengage and adapt themselves to each other so as to form a single homogeneous mass. This is obtained by drilling a series of spaced circular or elongated holes, which holes are filled on the spot with cement or the like material carrying a magnetic additive.
Primary elements are formed by drilling one or more primary holes in spaced relation, and then filling these holes with concrete containing magnetic particles, in order to be able to drill intermediate holes intersecting or overlapping the concrete poles already present in the first mentioned primary hole or holes and the concrete then filled into the intermediate holes intimately binds itself to the concrete in the first mentioned primary holes.
The problem solved by this feature of the present invention is to verify a minimum section of overlap at the joint of two adjacent elements in deep cut offs. Briefly the problem can be solved as follows:                After the completed excavation of primary elements, be it in the form of circular piles or in elongated shapes excavated by Hydromills, the element is filled with concrete or plastic concrete containing in the mix a magnetic substance (iron filings, steel fibers, magnetite sand and the like or the equivalent).        After the excavation of the secondary element, which has accomplished the task of forming a joint with the primaries by removing a portion of the adjacent primaries, a borehole probe is lowered into the excavation which will record the portion of the periphery of the hole with magnetic reading.        Since alignment is not important in deep cut-offs, but only continuity and minimum section, orienting the probe is not critical, since as long as it reads two minimum overlaps in the adjacent primaries, the purpose of the cut-off is achieved. In the absence of two minimum overlaps, remedial action may be taken, such as enlarging the borehole for the secondary wall element to achieve the desired overlap or drill a further overlapping hole and fill with concrete to correct for the misalignment and eliminate the discontinuity.        By taking the readings at preset intervals, minimum overlap for the whole depth can be verified.        